The sol—gel transformation of agarose and other polysaccharide fractions from agar were investigated by the fluorescence polarization method in order to study the gelling mechanism of agarose. The results showed that the gelation in dilute systems was a cooperative process of a helix segment with a kink segment in the agarose. In the process, the role of the helix segment was interpreted on the basis of Flory’s mechanism for the transition and phase equilibria of asymmetric polymers. That is, the helix segment folded into a rodlike helical chain at the helix point to form an isotropic solution. At the gel point, the isotropic solution was separated into the tactoidal concentrated solution phase (coacervate) and the isotropic more dilute solution phase. The coacervate droplets were joined to each other with the non-helical kink segment to form a three-dimensional network structure. The rodlike helical chain tended to aggregate into the crystalline phase, though the rate was fairly slow just below the helix point because of the nucleation controlled process. Some of the helical chains still tended to aggregate into the crystalline phase in the coacervate. The melting point of the gel corresponded to that of the crystalline phase, and was higher than the gel setting point. The hysteresis of the agarose sol—gel transformation was thus interpreted by the difference between the melting point and the setting point.